020 MK. K. II. WHITEHOUSE OX THE [Apr. 5, 



tho\ijrli tliis is soniewliat cleft di.stally ; this sti'uetuve is tlie sole 

 support of the eight caudal rays ; the last vertebral segment has 

 become most completely fused with the hypural. The whole fin 

 presents a most compact and solid structure. The neural and 

 h:emal spines form an imbricate series, each overlapping the one 

 behind. 



Plectogxathi. 



Balistes CAPRiscus. (Sclerodermi.) (Plate L. fig. 33.) 



No trace of a urostyle remain.s, and two large hypural liones 

 are firmly united with the last centrum. For such an advanced 

 condition of homocercy an irregularity occurs here, for the pen- 

 ultimate vei-tel)ra possesses an epural bone {cp.) instead of the 

 usually greatly reduced neural arch ; the penultimate vertebra 

 also bears a hypural bone. Two dorsal caudal radials (d.c.r.) are 

 <rlosely AVedged between the epural and last hypural, and a ventral 

 caudal radial {v.cr.) is found between the fi.i-st and second hypural. 

 The whole fin presents an apparent synnnetry. 



4. General Co^"CLUSIO^^s, 



There is eveiy reason for believing that the most specialized 

 caudal fins have been evolved from a common primitive type, 

 which is referred to as protocei-cal ; modifications of this produced 

 the heterocercal form, which in turn was followed by the homo- 

 cereal type. 



Protocercy. — A protocerciil fin may be defined as one which is 

 primitively symmetrical externally and intei-nally, and hence one 

 which has not luidergone any reduction or modification of the 

 original form. It is veiy probable that the protocercal tail is not 

 found in the adult condition among any living fishes. The Oyclo- 

 stomes are sometimes credited with having a protocercal caudal, 

 but it may be that it is a degenerate condition, and not a per- 

 sistent original structure. Since the original protocercal caudal 

 fiu was continuous with the dorsal and anal fins, and thus 

 luidiflerentiated, it is not unreasonable to suppose that the con- 

 tinuous fin was supported similarly throughout, i. e. by a series 

 of radials. 



It must not be supposed, however, that piotocercy is always 

 associated with a continuous median fin ; during development in 

 'I'eleo.sts the elements of the permanent difterentiated median fins 

 are laid ilown before heterocercy is estal)lished, so that there is 

 evidence for supposing that differentiation of the median fins 

 occurred in adult forms before heterocercy was adopted. 



IIin'EKOCERCV. Confining attention to typical forms, the 

 heterocercal caudal fin may be defined as one in which the 

 extremity of the chorda is directed upwards, and which as a 

 consec|uence is lULsymmetrical, the lower lobe being larger than 

 the upper, and. moreover, one in which, when centra are formed, 

 these elem(M\ts remain ilislinct 1o the end of the axis. 



